1. Field of the Invention
The present invention relates to a process of forming images on a printing form and, more particularly, to a process of forming images on a printing form wherein the printing form includes a first layer containing a ferroelectric material exhibiting the photoelectric effect, i.e. the photoferroelectric effect. Free charge carriers are generated in the ferroelectric material by irradiation with light above the photoelectric threshold frequency of the ferroelectric material. The free charge carriers may also be generated by the photoelectric effect in a nonferroelectric layer adjoining the ferroelectric layer on which images are to be formed, this layer being solely a charge-generating layer of a photoconductor formed of a plurality of layers and is therefore not a photoconductor in the conventional sense. Further, the invention is directed to a printing form including a ferroelectric layer especially designed to enhance the photoferroelectric effect.
2. Description of the Prior Art
A printing form which is constructed as a thin disk or plate with a ferroelectric material and a photoconductive coating on one of its surfaces is known from German Patent Application DT 25 30 290 A1. A first electrode is arranged surfacewise below the ferroelectric material and a second electrode is applied to the photoconductive coating. The photoelectric layer acts as a switch. At least one of the two electrodes is removable and at least the electrode on the photoconductive coating is transparent to light. If an optical image is focussed on the photoconductive coating of the printing form and an electric voltage is applied to the two electrodes at the same time, the ferroelectric material can be polarized imagewise. Instead of beaming the image onto the surface of the printing form with a master image and by focussing light the surface of the printing form can also be scanned by a focussed light beam, e.g. a laser beam. If a d.c. voltage is applied to the two electrodes at the same time, the specific electric resistance of the photoconductive coating decreases in the light regions on the photoconductive surface, i.e. in the regions exposed imagewise. Therefore, the d.c. voltage acts principally on those regions of the ferroelectric material which lie below the light image regions of the photoconductive coating. The specific resistance of the photoconductive coating remains high in the dark image regions. Accordingly, a ferroelectric polarization is induced in the ferroelectric material only in those regions corresponding to the light image regions.
A printing process using a pyroelectric film is known from U.S. Pat. No. 3,899,969. Ferroelectric materials, e.g. lead-zirconate-titanite or polyvinylidene fluoride, are also used for this pyroelectric film. A ferroelectric material of this type is introduced, for example, between two surfacewise electrodes, one of which is transparent to light. A voltage is applied between the two electrodes and the ferroelectric film is selectively heated by electromagnetic radiation in accordance with an image pattern to be formed on it. The ferroelectric material is permanently polarized in accordance with the image by surfacewise application of the electric field and by selective heating. The cumbersome photothermal effect which is not always controllable is used for this purpose.